Constant power supply for LED emergency lighting using smart output resetting circuit for no load conditions

ABSTRACT

A constant power backup power supply for LED lighting fixtures is disclosed. The power supply includes a storage battery that is charged while an AC power source is in an ON condition. When AC power transitions to an OFF condition, a capacitor bank charged by the battery supplies current to the primary side of a flyback converter operating in discontinuous conduction mode. The secondary side of the flyback converter supplies constant output power to the LED lighting fixture for an arbitrary output voltage within a predetermined range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/213,155, filed Jul. 18, 2016, which is a continuation of U.S. patentapplication Ser. No. 13/866,951, filed Apr. 19, 2013, which issued asU.S. Pat. No. 9,398,649 on Jul. 19, 2016, the disclosures of which areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The invention relates to emergency backup power supplies for LEDillumination. In particular, the invention relates to an emergencybattery backup system for driving LED illumination fixtures on aconstant power basis, enabling the support of LED illumination fixtureshaving arbitrary voltage drops.

BACKGROUND OF THE INVENTION

Illumination based on light emitting diodes (“LEDs”) has a variety ofadvantages over conventional incandescent and fluorescent illumination,for both commercial and residential settings. LED lighting is inherentlymuch more efficient than incandescent illumination since virtually allof the optical power emitted by LED emitters is distributed in thevisible spectrum. While fluorescent bulbs (including compact fluorescentor “CFL” bulbs) are comparatively more efficient than incandescentbulbs, the spectrum of light emitted by fluorescent sources is stillviewed as unpleasant as it fails to mimic the spectrum of sunlight.Additionally, fluorescent light sources (like incandescent sources) arefragile, requiring glass envelopes to maintain a vacuum and/or tocontain the discharge gas. In the case of CFL bulbs, these bulbs containtrace amounts of mercury, which can be released when the bulbs break.Additionally, fluorescent lights take more time to fully illuminate.

LED lighting has the potential to overcome all of these disadvantageswith conventional lighting sources. As is set forth above, LEDs areinherently efficient. Additionally, “white” LEDs, which are constructedby providing a short wavelength LED source which excites a phosphoremitter, can be “tuned” and then filtered to achieve a variety ofpleasing color temperatures. LEDs are mechanically sturdy. AdditionallyLEDs can be driven in DC mode, eliminating flicker.

Given the advantages of LED illumination, LED illumination fixtures areincreasingly being used in commercial and/or industrial environments. Asa result of various statutory and practical safety requirements,lighting fixtures used in commercial and industrial settings generallymust be equipped with a backup system to provide a minimum level ofillumination in the event of a power failure. Conventional batterybackup systems for driving fluorescent illumination fixtures areunsuitable for use with LED fixtures, which have vastly differentcurrent, voltage and drive characteristics.

Conventionally available LED lighting fixtures are provided as seriescombinations of LEDs, typically driven in DC. LEDs have I-Vcharacteristics that are similar to other semiconductor diodes, which isto say, that current varies exponentially as a function of forwardvoltage. The “white” LEDs used in commercial lighting applicationstypically drop 3.3 to 4.5V at their ideal operating forward current.Commercially available DC LED lighting fixtures are available in 12 VDC,20 VDC and 30 VDC, representing different numbers of series connectedLEDs. Conventional emergency backup drivers for DC LED lighting fixturesare provided as constant current devices, for example, by varying loadpower. However, because DC LED lighting fixtures require a variety ofdifferent operating voltages, a variety of conventional emergency backupdrivers are required to supply the necessary current at the voltagerequired by the specific lighting fixture to be driven. It would bedesirable to provide an emergency backup LED driver capable of supplyingbackup power to LED lighting fixtures regardless of forward voltage.

SUMMARY OF THE INVENTION

Embodiments of the invention include a constant power battery backupsystem for driving LED illumination fixtures in DC. Embodiments of theinvention ensure constant power delivery to LED fixtures havingarbitrary voltage requirements by using a discontinuous conducting mode(DCM) flyback converter. Systems according to the present invention arecapable of supplying backup battery power on a constant power basis atthe high DC currents required by commercial LED lighting fixtures, i.e.,100-1000 mA.

Other aspects of the invention include a DCM flyback converter operatingin conjunction with a low battery sense circuit, which protects a backupbattery source from over depletion during operation of the LED backuplighting system according to the invention.

Other aspects of the invention include a no load resetting circuit,which regulates the output voltage of the system of the invention whenthe LED load to be driven is disconnected from backup driver.Additionally, the no load resetting circuit according to theaforementioned aspect of the invention limits LED inrush current toprevent damaging the LED load at low voltages.

In a first embodiment, a backup power supply for driving an LED lightsource is provided. The supply includes a storage battery adapted toprovide DC electrical current and a constant power LED driver circuithaving a flyback converter having a transformer operating indiscontinuous conduction mode. The flyback converter has a primaryinductor winding that is electrically coupled to the storage battery,and a secondary inductor winding that is selectably electrically coupledto the LED light source.

In another embodiment, the backup power supply has a capacitor bankelectrically interposed between the primary inductor winding and thestorage battery. The capacitor bank has a combined capacitancesufficient to supply sufficient current to the primary inductor windingto supply constant power to the LED light source over a predeterminedoutput voltage and power range.

In yet another embodiment, the backup power supply further includes aPWM controller electrically coupled to gate voltage and drain currentfrom the capacitor bank to the primary inductor winding of the flybackconverter with a square wave signal having a frequency and pulse width.The frequency and pulse width of the square wave signal, and the designof the transformer are such that that primary inductor winding does notsaturate during the application of the square wave signal.

In another embodiment, the backup power supply also has a smartoutput/no load resetting circuit electrically coupled to the secondaryinductor and an output capacitor. The smart output/no load resettingcircuit is configured to discharge the output capacitor through aresistive load in the event that the LED light source is decoupled fromthe backup power supply.

In another embodiment, the backup power supply is electricallyinterposed between an AC current source and the LED light source, andthe backup power supply further includes an emergency LED driver andexternal AC LED driver switcher circuit that electrically couples the ACcurrent source to the LED light source when the AC current source is inan ON condition, and which alternatively electrically couples the LEDdriver circuit to the LED light source when the AC current source is inan OFF condition.

In yet another embodiment, the backup power supply has an AC on delaycircuit electrically interposed between said emergency LED driver andexternal AC LED driver switcher circuit and said LED light source. TheAC on delay circuit is configured to supply a delay to the supply of ACcurrent to the LED light source when the AC current source transitionsfrom an OFF to an ON condition.

In one embodiment, the backup power supply further includes an AC inputbattery charger electrically coupled to an AC current source and abattery. The AC input battery charger is configured to charge saidbattery when said AC current source is in an ON condition.

In yet another embodiment, the backup power supply has a low batterydrop circuit electrically coupled between the constant power LED drivercircuit and the battery. The low battery drop circuit is adapted tosense an output voltage of the battery and disconnect the constant powerLED driver circuit from the battery when the battery output voltagedrops below a predetermined level.

Further embodiments of the invention include methods of providingcomponents and circuits set forth in the preceding paragraphs.

In another embodiment, the invention includes a flyback converter forproviding constant power to an LED load. The flyback converter isconfigured to supply sufficient DC current to maintain a predeterminedconstant output power over a range of between 20 and 50V.

In another embodiment, the flyback converter is designed for apredetermined constant output power is one of 5, 7 or 10 W.

Aspects of the invention have certain advantages over conventionalsystems. Embodiments of invention allow for constant power driving ofarbitrary arrays of LEDs, within broad ranges of operating voltages,despite the specific operating voltage of a given driven array. Thisallows the same constant power backup system to be used on differenttypes of LED lighting fixtures having different operating voltages.Embodiments of the invention are configured to deliver constant powerover a lighting fixture operating range of from 20 to 48 VDC.

Embodiments of the invention are highly efficient, supplying up to 80%of input power to the LED lighting fixtures. Additionally, becauseoutput power, and therefore input power, is constant, the power drawdownfrom the backup batteries provided in the system is constant. Thisallows for batteries to be predictably drawn down to 10% of their storedpower in the 90 minute backup period required by commercial standards,which in turn, allows for more efficient use of battery capacity.

Additionally, embodiments of the invention guard against dangerousconditions present when AC power is lost by the LED array and the backupsource is connected, or vice versa. In particular, aspects of theinvention use a “smart reset circuit” to overcome dangerously highvoltage on the output of the backup system during open load conditions,prior to connection of the LED load. Additionally, the system isprotected if the voltage load presented by the LED is out ofspecification.

Additionally, embodiments of the invention include a low battery shutdown or battery drop circuit to protect the battery from over depletionand to guarantee constant output power within the operating voltage ofthe battery.

Additional advantages will become clear throughout the followingdetailed description of the preferred embodiments and in viewing theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an exemplary constant power backup supplyfor LED lighting according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention is described in preferred embodiments in the followingdescription with reference to the Figures, in which like numbers orindications represent the same or similar elements. Referencesthroughout this specification to “one embodiment,” “an embodiment,” “arelated embodiment,” or similar language mean that a particular feature,structure, or characteristic described in connection with the referredto “embodiment” is included in at least one embodiment of the presentinvention. Thus, appearances of the phrases “in one embodiment,” “in anembodiment,” and similar language throughout this specification may, butdo not necessarily, all refer to the same embodiment. It is to beunderstood that no portion of disclosure, taken on its own and inpossible connection with a figure, is intended to provide a completedescription of all features of the invention.

The described features, structures, or characteristics of the inventionmay be combined in any suitable manner in one or more embodiments. Inthe following description, numerous specific details are recited toprovide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventionmay be practiced without one or more of the specific details, or withother methods, components, materials, and so forth. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

FIG. 1 is a schematic circuit diagram showing a constant power backupsupply for LED lighting according to an embodiment of the invention. Inthe embodiment of FIG. 1, the circuit of FIG. 1 is interposed between anAC power source, connected to connector H1, and an array of seriesconnected LEDs (i.e., a lighting fixture described below as an LEDload), connected to connector H2. The embodiment of the inventiondepicted in FIG. 1 involves the combination of 7 logically andphysically separable sub-circuits, labeled Circuit 1 through Circuit 7in FIG. 1 and interconnected as shown. While certain embodiments of theinvention include each of Circuits 1 through 7, others do not, andinclusion of each of Circuits 1 through 7 in FIG. 1 should not be deemeda limitation on the invention.

In the embodiment of FIG. 1, Circuit 1 provides a universal AC inputbattery charger. As can be seen in FIG. 1, this functionality isprovided by a flyback converter (comprising variable transformer T2 withtaps as shown) operating in conjunction with non-illustrated AC powersupply, PWM controller U1, and the illustrated passive components. Ascan be seen in FIG. 1, Circuit 1 comprises components H1, VR1, D1, C1,C2, R30, C7, U1, C4, C5, C13, R1, R2, D3, D2, R4, R17, C18, T2, C14,R15, D4, C15, C16, C17, R16 and F1. The flyback converter of FIG. 1operates in discontinuous conducting mode, supplying continuous outputcurrent to non-illustrated batteries while AC power is supplied to thesystem, i.e., while AC power is in an ON condition. An exemplary PWMcontroller is the FSEZ1317NY Primary-side-regulation PWM with PowerMOSFET available from Fairchild semiconductor.

In a particular embodiment, VR1 is an ESD protection diode, F1 is a 1 Afuse, and D1 is a bridge rectifier. In the embodiment of FIG. 1, thecapacitances of the capacitors of Circuit 1 are as follows: C2, C4 andC17 are 0.1 uF, C1 and C5 are 10 uF, C7 is 1 uF, C13 is 33 pF, C18 is680 pF, C14 is 4700 pF, C15 is 1000 pF, and C16 is 100 uF. Theresistances of the resistors in Circuit 1 in the embodiment of FIG. 1are as follows: R30 is 100KΩ, R1 is variously 1.21Ω or 1.33Ω dependingon designed for output power, R2 is variously 17.4 k Ω or 18.2 k Ω, R4is 47.5KΩ, R17 is 200 KΩ, R15 is 47.5Ω, R 16 is 20.0KΩ.

During the presence of Vac (108-304 Vac) the Universal AC Input BatteryCharger circuit provides power (˜37 Vdc across R16) to Circuit 2, the ACpower on delay circuit, Circuit 3, the emergency LED driver and externalAC LED driver switcher, and charges non-illustrated battery connected atBAT + and BAT −. When no Vac is applied to Circuit 1, i.e., when ACpower is in an OFF condition as occurs during a power failure, bothCircuit 2, the AC power on delay circuit” and Circuit 3, the emergencyLED driver and external AC LED driver switcher are turned off. Circuit4, which is a capacitor bank and filter capacitor, Circuit 5, the lowbattery drop circuit, Circuit 6, the constant power LED driver, andCircuit 7, the smart output and no load resetting circuit, are suppliedby non-illustrated battery connected at BAT+ and BAT− Battery (BAT+, BAT−) when no Vac is applied to Circuit 1, i.e., when AC power is in an OFFcondition. The operation of these Circuits is set forth in greaterdetail below.

The embodiment of FIG. 1 also includes AC power on delay circuit,Circuit 2. As can be seen in FIG. 1, Circuit 2 comprises components R21,R20, C23, Q3, D10, D11 and K5. The function of AC power on delay circuitis to delay the reapplication of AC power to the external AC LED driverafter a period of emergency backup power use, i.e., when the AC powerinput transitions from an OFF to an ON state. In particular, AC power ondelay circuit, Circuit 2, controls the “Dly1” and “Dly2” which areneutral lines coupled to an external AC Driver. When power is present atCircuit 2, the current flows thru transistor Q3, thereby preventingrelay K5 rom switching for a short period of time after reapplication ofAC power. In one embodiment the length of the delay is approximately 2seconds. As capacitor C23 (330 uF) charges, the voltage at the base ofQ3 increases to where Q3 turns off allowing current flow thru relay K5(PIN2, PIN5). Once K5 is energized it makes the contact between CON(PIN1) and NO (PIN3) after a brief delay. The magnitude of the delay canbe controlled by selection of R20 and R21, which is one embodiment arerespectively 49.9KΩ and 2.26KΩ resistors, and capacitor C23, which inone embodiment is 330 uF. When “Dly1” and Dly2″ make contact, via K5,the external AC Driver is energized.

The embodiment of FIG. 3 also includes Circuit 3, which is the emergencyLED driver and external AC LED driver switcher circuit. As can be seenin FIG. 1, Circuit 3 comprises components D12, R33, R3, K1, K2, K3 andH2. Circuit 3 selectably connects either the external AC power source(in the event of an AC ON condition), or constant power emergency LEDdriver (in the event of an AC OFF condition) to the LED array.

In particular, Circuit 3 controls the connection between the LED loadand the non-illustrated battery. When power is applied to the coils ofK1A, K2A, and K3A the contacts K1B, K2B, and K3B switch from CON-NC(PIN1-PIN4) to CON-NO (PIN1-PIN3). The positive and negative connectionsto the LOAD are controlled by K2B (positive) and K3B (negative). Theload is connected, via H2, to CON (PIN1) of K2B and to CON (PIN1) ofK3B. The external AC LED driver output (positive and negative) isconnected to NO (PIN3) of K2B and to NO (PIN3) of K3B. The emergency LEDdriver (Circuit 6) is connected to NC (PIN4) of K2B and NC (PIN4) ofK3B. During the time the coils K2A and K3A are energized the external ACLED driver powers the LOAD via K2B and K3B (PIN1-PIN3). When K2A and K2Bare de-energized the LOAD is powered by the Emergency LED driver(Circuit 6) via K2B and K3B (PIN1-PIN4). The Battery (BAT+) iscontrolled by K1A. During the time the coil K1A is energized theconnection between “GO1” and “GO2” is broken via K1B (PIN1-PIN3)allowing current from universal AC input battery charger (Circuit 1) tocharge the battery. When K1A is de-energized the connection between“GO1” and “GO2” is made via K1B (PIN1-PIN4) allowing the battery topower the capacitor bank and filter capacitor (Circuit 4), low batterydrop circuit (Circuit 5), constant power LED driver (Circuit 6) andsmart output no load resetting circuit (Circuit 7), the operation ofwhich are described in additional detail below.

The embodiment of FIG. 1 also includes Circuit 4, which is a capacitorbank electrically coupled to the non-illustrated battery. As can be seenin FIG. 1, Circuit 4 comprises components C9, C24, C25 and C12. Thecapacitor bank of Circuit 4 includes source capacitors C9, C24 and C25,which store charge to be discharged in driving the constant power LEDdriver, set forth in additional detail below. Capacitor C12 (0.1 uF) isa filter capacitor intended to filter high frequency noise fromswitching MOSFET transistor Q1 and Flyback transformer T1. In oneembodiment, each of C9, C24 and C25 comprises 820 uF polymer capacitorsselected to have low ESR ensuring their ability to supply maximum inputcurrent to the primary inductor for the transformer T1 in Circuit 6without distortion. In particular, when battery voltage is applied tothis Circuit 4 via “GO2” the capacitors C9, C24, C25, and C12 getcharged ensuring source capacitors' ability to supply current up to themaximum peak current that can be driven through the primary coil oftransformer T1 without distortion.

The embodiment of FIG. 1 also includes Circuit 5, which is a low batterydrop circuit. As can be seen in FIG. 1, Circuit 5 comprises componentsR19, C11, R24, R23, R26, R25, D13, U3, R28, R22 and Q5. Circuit 5 iscontrolled by non-illustrated battery through connection to BAT +, BAT−. Circuit 5 monitors the battery voltage and controls the power goingto the “constant power LED driver”. When battery voltage is applied tothis circuit via “GO2” the output of amplifier U3A goes “low” allowingtransistor Q5 to turn on. When transistor Q5 is on, controller U2 ispowered on allowing Circuit 6, the constant power LED driver to operate.When the battery voltage drops below 87.5% of nominal voltage the outputof amplifier U3A goes “high” turning off Q5. When Q5 is off controllerU2 is powered off, resulting in Circuit 6 being powered off. In oneembodiment, the values of the passive components of Circuit 5 are asfollows: R19 is 825Ω, C11 is 1 uF, R24 is 1.69K Ω, R23 is 1.78K Ω, R26is 475Ω, R25 is 1.91KΩ, R28 is 10 KΩ, and R22 is 3.57KΩ

The embodiment of FIG. 1 also includes Circuit 6, which is the constantpower LED driver circuit. As can be seen from FIG. 1, Circuit 6comprises components C20, C21, R10, R11, R6, R14, C22, R5, U2, R8, D5,D9, Q1, R9, R12, T1, D8, C19 and C10. The constant power LED drivercircuit of Circuit 6 comprises a discontinuous conduction mode (DCM)flyback converter, which in turn comprises transformer T1. The primarywinding inductance of T1 is selected to not limit the peak inductancecurrent which the current saw wave remains linear. T1 includes a corematerial selected such that its flux density does not saturate withinthe designed constant power operating range, i.e., within the peakcurrent range required to support the designed-for output power range.Variations in input power to the primary winding of T1 (created by, forexample, variations in battery voltage) are controlled by adjusting thepulse width of the input signal driving T1 in conjunction withcontroller U2. The value of R11 is large enough to achieve a high gainwhile having a quick response to an output over voltage condition. TheR14 value is selected to compensate for input voltage variation as afeed forward and R12 is provided in parallel with R9 to provide finetuning for input power regulation. Resistors R10 and R6 limit the outputvoltage to 50V in the event that the load is disconnected. This isrequired by statutory and safety standards.

To operate in constant power, the flyback converter operates indiscontinuous conducting mode in order to deliver all energy, which isstored in primary winding of transformer T1, to the secondary in eachswitching cycle. If the peak current of primary inductor is capped thenthe turn on time will increase, preventing discharge of all of theenergy stored in the primary coil. In that scenario the converterbecomes continuous conducting mode operating then in each switchingcycle the energy transfer between primary and secondary does notcomplete.

Additionally, in the embodiment of FIG. 1, the primary side power isregulated without feedback from secondary, and the driving parametersare chosen to guarantee complete energy transfer from the primarywinding of transformer T1 to the secondary. This guarantees that Circuit6 operates in constant power. In particular, because the flybackconverter of Circuit 6 operates in discontinuous mode, and as long asthe primary input current to transformer T1 is limited to be below thepeak current supportable by T1 s primary inductor, Circuit 6 guaranteesthat the output power supplied to the LED load is dependent only on thepeak inductor current (Ip), and is completely independent of voltage inand voltage out. This is a key feature of the invention: output power isindependent of the Vout (i.e., the voltage associated with arbitrary LEDloads), so long as the circuit can supply sufficient output current.Assuming no loses, Pin to Circuit 6=Pout=½*L*lp^2, where L is 5.4 uH.

The design parameters of T1 are given by Faraday's law: Vin=4*n*Bm*Ac*fsand Lp=2*Vin^2/(Pin*fs), where n is the number of primary turns on theprimary coil of T1, Vin is DC battery voltage (which in one embodying isin the range of 9-10V), Bm is flux density, Ac is core cross section, fsis switching frequency (of the input signal to T1), Pin is input powerand Lp is primary inductance. In the embodiment of Circuit 6, sensingresistors R10 and R6 are for over-voltage protection only. In order toensure that Circuit 6 operates in discontinuous conducting mode, thecircuit is designed to run maximum peak current under an input DCbattery voltage of 9.6V. In one embodiment, 5 turns are used for theprimary of T1, and three 820 uF bulk capacitors are used on the inputside to guarantee the ability of Circuit 6 to supply between 5 W and 10W of output power for any output voltage up to 50V. If input batteryvoltage change and output power change we need change transformerwinding turns and capacitor's value accordingly.

Applicant has produced 3 prototype constant power LED drivers accordingto the embodiment of FIG. 1: a 5 W, a 7 W and a 10 W prototype, each ofwhich is operational over a Vout range of approximately 20-50V. For the5 W prototype, the values of the passive components of the circuit ofFIG. 1 are as follows: R2 is 17.4 KΩ, R9 is 1.21Ω, R12 is 0.250Ω andR1=1.33Ω. For the 7 W prototype the values of the passive components ofCircuit 6 are as follows: R2 is 18.2 KΩ, R9 is 1.78Ω, R3 and R33 are715Ω, R12 is 0.180Ω and R1=1.21Ω. For the 10 W prototype the values ofthe passive components of Circuit 6 are as follows: R2 is 18.2 KΩ, R9 is3.24Ω, R3 and R33 are 715Ω, R12 is 0.150Ω and R1=1.21Ω. The values ofthe passive component of Circuit 6 that are common for all designed-forpower levels are as follows: R10 is 38.3 KΩ, R11 is 1.0 MΩ, R6 is2.00KΩ, R5 is 10 KΩ, R8 is 1.00KΩ, C20 is 0.1 uF, C21 is 33 pF, C22 is1000 pF, C19 is 0.1 uF and C10 is 56 uF.

The embodiment of FIG. 1 further includes Circuit 7, which providesfault protection, specifically, a smart output/no load resettingcircuit. As can be seen in FIG. 1, Circuit 7 includes components R29,Q6, DZ1, R31, R32 and DZ2. This portion of the circuit of FIG. 1 iscontrolled by non-illustrated battery connected to the circuit of FIG. 1through terminals BAT +, BAT −. Circuit 7 drains/discharges thecapacitor bank of Circuit 4, as well as C19 and C10 on Circuit 6, whichprotects the LED load if it is disconnected and reconnected duringoperation of the constant power LED driver circuit of Circuit 6. Inparticular, when battery voltage is present via “GO2” the Gate oftransistor Q6 has this voltage present. The gate voltage of Q6 isreverse biased compared to the 12V of DZ1 on the source of Q6. As aresult, in this condition, Q6 is off and resistor R29 (2K Ohm) isdisconnected from output of the constant power LED driver of Circuit 6to avoid resistor R29 draining battery power in emergency operation(i.e., in an AC power OFF condition).

Another function of Circuit 7 is to reset the voltage on C10 such thatit drops to less than 20V within 0.3 seconds of the LED load beingdisconnected from the driver. In particular, when both the LED load andbattery power are disconnected, charge remains stored in bulk capacitorsC9, C24 and C25 while the constant power LED driver circuit remainsrunning, which causes the voltage on C10 to increase to 50V. However,when the Vgs of Q6 becomes forward biased by the absence of batteryvoltage on the gate of Q6, Q6 is on and R29 is connected to the outputof the constant power LED driver. The charge in C10 is then quicklydischarged through R29 to be below 20V.

Thus, according to the design described above, when the DC batterypresents its voltage, the resetting circuit is disconnected from theoutput circuit by Q6 to prevent power loss. When the LED load switchesback to connect the driver, the inrush LED current is limited to preventdamaging the LED load.

The invention has been described with respect to particular embodiments.Those having skill in the art will recognize that additional embodimentswill be within the scope of the invention. The invention is not limitedby the description of particular embodiments, but instead, is defined inaccordance with the following claims.

The invention claimed is:
 1. A backup power supply for driving an LEDlight source, the supply comprising: a storage battery adapted toprovide DC electrical current; a constant power LED driver circuitcomprising a flyback converter comprising a transformer having a primaryinductor winding that is electrically coupled to the storage battery,and a secondary inductor winding that is selectably electrically coupledto the LED light source; and further comprising one or more capacitorselectrically interposed between the primary inductor winding and thestorage battery, the one or more capacitors having a capacitancesufficient to supply sufficient current to the primary inductor windingto supply constant power to the LED light source over a predeterminedoutput voltage and power range.
 2. The backup power supply of claim 1,wherein the flyback converter further comprises a PWM controller.
 3. Thebackup power supply of claim 2, wherein the PWM controller iselectrically coupled to gate voltage and drain current from the one ormore capacitors to the primary inductor winding of the flyback converterwith a square wave signal.
 4. The backup power supply of claim 3,wherein the square wave signal has a frequency and a pulse width, andwherein the frequency and pulse width of the square wave signal, and thedesign of the transformer are such that that primary inductor windingdoes not saturate during the application of the square wave signal. 5.The backup power supply of claim 1, wherein the transformer isconfigured to operate in discontinuous conduction mode.
 6. The backuppower supply of claim 1, further comprising one or more outputcapacitors electrically coupled between a secondary inductor winding ofthe transformer and the LED light source.
 7. The backup power supply ofclaim 6, further comprising a resistive load coupled to the one or moreoutput capacitors.
 8. The backup power supply of claim 7, furthercomprising a smart output/no load resetting circuit electrically coupledto the secondary inductor and the output capacitor, wherein the smartoutput/no load resetting circuit is configured to discharge the outputcapacitor through the resistive load in the event that the LED lightsource is decoupled from the backup power supply.
 9. The backup powersupply of claim 1, wherein the backup power supply is electricallyinterposed between an AC current source and the LED light source, andwherein the backup power supply further comprises an emergency LEDdriver and external AC LED driver switcher circuit that electricallycouples the AC current source to the LED light source when the ACcurrent source is in an ON condition, and which alternativelyelectrically couples the LED driver circuit to the LED light source whenthe AC current source is in an OFF condition.
 10. The backup powersupply of claim 9, further comprising an AC on delay circuitelectrically interposed between said emergency LED driver and externalAC LED driver switcher circuit and said LED light source, wherein saidAC on delay circuit is configured to supply a delay to the supply of ACcurrent to the LED light source when the AC current source transitionsfrom an OFF to an ON condition.
 11. The backup power supply of claim 1,further comprising an AC input battery charger electrically coupled toan AC current source and said battery, wherein the AC input batterycharger is configured to charge said battery when said AC current sourceis in an ON condition.
 12. The backup power supply of claim 1, furthercomprising a low battery drop circuit electrically coupled between theconstant power LED driver circuit and the battery, wherein the lowbattery drop circuit is adapted to sense an output voltage of thebattery and disconnect said constant power LED driver circuit from thebattery when the battery output voltage drops below a predeterminedlevel.
 13. A method of providing backup power to an LED light source,the method comprising: providing a storage battery adapted to provide DCelectrical current; and providing a constant power LED driver circuitcomprising a flyback converter comprising a transformer, wherein theflyback converter has a primary inductor winding that is electricallycoupled to the storage battery, and the flyback converter has asecondary inductor winding that is selectably electrically coupled tothe LED light source; and further comprising providing one or morecapacitors electrically interposed between the primary inductor windingand the storage battery, the one or more capacitors having a capacitancesufficient to supply sufficient current to the primary inductor windingto supply constant power to the LED light source over a predeterminedoutput voltage and power range.
 14. The method of claim 13, furthercomprising providing a voltage and current from the one or morecapacitors to the primary inductor winding of the flyback converter. 15.The method of claim 14, wherein the voltage and current are provided tothe primary inductor winding of the flyback converter as an AC signalhaving a frequency and pulse width, and wherein the frequency and pulsewidth of the square wave signal, and the design of the transformer aresuch that that primary inductor winding does not saturate during theprovision of the square wave signal.
 16. The method of claim 13, whereinthe predetermined output voltage comprises a predetermined outputvoltage range.
 17. The method of claim 16, wherein the predeterminedoutput voltage range is up to 50V.
 18. The method of claim 13, whereinthe predetermined constant output power is in the range of between about5 and 10W.
 19. The method of claim 13, further comprising operating theflyback converter in discontinuous conduction mode.